America’s 20 dirtiest cities

California may have the nation's first cap-and-trade program, but it also has seven of the 10 most polluted cities, according to Forbes. Here are the rest of the 20 dirtiest metropolitan areas:

20. Cleveland, Ohio

19. Indianapolis, Ind.

18. Houston, Texas

17. Salt Lake City, Utah

16. Washington, D.C.

15. Knoxville, Tenn.

14. New York, N.Y.

13. Cincinnatti, Ohio

12. Philadelphia, Pa.

11. Louisville, Ky.

10. Sacramento, Calif.

9. San Diego, Calif.

8. Phoenix, Ariz.

7. Birmingham, Ala.

6. Modesto, Calif.

5. Pittsburgh, Pa.

4. Fresno, Calif.

3. Visalia, Calif.

2. Los Angeles, Calif.

1. Bakersfield, Calif.

Most of these offenders have actually improved a lot in the last 10 years — New York, for instance, used to average 40 bad ozone days a year, and now has only 17. Even Bakersfield, winner of the extremely corroded gold medal, has made great strides, although it's still a terrible place for breathers:

Hot, dusty, adjacent to California’s biggest oil fields, Bakersfield has 60 days a year of unhealthy air, 10 times a level considered acceptable. Its ozone levels are better than at any time in the past 15 years, but still unhealthy for 100 days out of the year.